To size a 3-phase dry-type transformer, calculate your total connected load in kW, divide by the expected power factor (assume 0.85 if unknown), and select the next standard kVA rating. For a standard 120/208V commercial panel, a 75 kVA transformer supplies exactly 208A on the secondary and requires a 125A primary breaker at 480V. This guide provides the exact lookup tables, derating math, and breaker pairings based on NEMA ST-20 and ANSI/IEEE C57.12.01 standards.
The Master kVA Transformer Sizing Chart (NEMA ST-20)
How to read this table: This chart assumes a standard 3-phase, 60Hz dry-type transformer with a 480V Delta primary and a 208Y/120V secondary. The Full Load Amps (FLA) are calculated at nominal voltage. Breaker sizes are selected based on NEC 450.3(B) rules for over 9A primary current (125% of FLA, rounded up to the next standard size) and standard secondary panelboard mains. Wire sizing assumes 75°C rated copper terminations, which is the default for most commercial breakers and transformer lugs unless explicitly marked 90°C.
| kVA Rating | Primary Voltage | Secondary Voltage | Primary FLA | Secondary FLA | Pri. Breaker (Max) | Sec. Breaker (Main) |
|---|---|---|---|---|---|---|
| 30 kVA | 480V Delta | 208Y/120V | 36.1 A | 83.3 A | 50 A | 100 A |
| 45 kVA | 480V Delta | 208Y/120V | 54.1 A | 124.9 A | 70 A | 150 A |
| 75 kVA | 480V Delta | 208Y/120V | 90.2 A | 208.2 A | 125 A | 250 A |
| 112.5 kVA | 480V Delta | 208Y/120V | 135.3 A | 312.3 A | 175 A | 350 A |
| 150 kVA | 480V Delta | 208Y/120V | 180.4 A | 416.4 A | 250 A | 500 A |
| 225 kVA | 480V Delta | 208Y/120V | 270.6 A | 624.5 A | 350 A | 700 A |
| 300 kVA | 480V Delta | 208Y/120V | 360.8 A | 832.7 A | 450 A | 1000 A |
Quick-Jump: Most Queried kVA Ratings & Wire Sizing
Transformers in the 45 kVA to 112.5 kVA range make up the vast majority of commercial and light industrial installations. Here are the specific wiring and termination details for the three most common sizes.
- 45 kVA (124.9A Secondary): Requires 1/0 AWG copper THHN (rated 150A at 75°C). Use a 150A secondary breaker.
- 75 kVA (208.2A Secondary): Requires 4/0 AWG copper THHN (rated 230A at 75°C). Do not use 3/0 AWG (200A), as it violates the continuous load rule. Pair with a 250A secondary breaker.
- 112.5 kVA (312.3A Secondary): A single 400 kcmil wire (335A at 75°C) works, but is stiff and hard to bend into the lugs. The better jobsite choice is paralleling two sets of 2/0 AWG copper (175A x 2 = 350A). Ensure your transformer lugs are explicitly rated for parallel conductors.
Decision Tree: Picking Your Exact Transformer & Derating Factors
The kVA chart above gives you the baseline size, but the nameplate specifications dictate whether it will survive in your specific environment. You must select the correct temperature rise and impedance columns, then apply derating factors if your installation falls outside standard conditions.
| Installation Condition | Required Action | Concrete Specification Pick |
|---|---|---|
| Standard linear load (heaters, incandescent, basic motors) in a climate-controlled room. | Select standard NEMA ST-20 dry-type. | 150°C rise, 5.75% impedance. |
| High non-linear load (>30% VFDs, LED drivers, server racks, SMPS). | Harmonics cause severe eddy current heating. Step up to a K-rated unit. | K-13 rated, 115°C rise, electrostatic shield. |
| Ambient room temperature exceeds 40°C (104°F). | Derate the kVA capacity or buy a larger physical frame. | Derate by 0.5% for every 1°C over 40°C, or select a 115°C rise unit. |
| Installation altitude is above 3,300 ft (1,000m). | Thinner air reduces cooling efficiency. | Derate kVA by 0.5% for every 1,000 ft above 3,300 ft. |
| High available fault current at the primary bus. | Limit the let-through current to protect secondary switchgear. | Specify 7.5% or 10% impedance (standard is 5.75%). |
Which column applies to you? If you are installing a transformer in a standard US commercial building with an HVAC system maintaining 75°F, the 150°C temperature rise column is your default. This means the transformer coils will reach 150°C above the 40°C ambient baseline (total 190°C) at full load, utilizing 220°C insulation class materials. If the room is unconditioned or houses heavy VFDs, you must specify a 115°C rise unit, which uses a larger copper core to run cooler.
What This Chart Cannot Tell You (And How to Calculate It)
A standard kVA sizing chart assumes ideal, steady-state conditions. Real-world jobsite physics will introduce three major variables that the table above ignores.
1. Magnetizing Inrush Current
When you first energize a dry-type transformer, the core magnetizes. This causes an asymmetrical inrush current that can spike to 10 to 12 times the primary FLA for the first half-cycle. If you use standard thermal-magnetic breakers on the primary side without checking the magnetic trip curve, the breaker will instantly trip upon energization. The fix: Ensure your primary breaker has a high instantaneous magnetic trip setting, or use a breaker specifically listed for transformer inrush (often denoted with a 'HID' or high-magnetic rating).
2. Voltage Regulation and Impedance Drop
The '5.75% impedance' listed on a standard nameplate isn't just for fault calculations; it's your voltage drop at full load. If you pull a continuous 208A from a 75 kVA transformer with 5.75% impedance, your secondary line-to-line voltage will drop from 208V down to roughly 196V. If your secondary feeder is long and adds another 3% drop, your end receptacles will measure below 190V, causing motors to overheat and IT equipment to brownout. The fix: Use the primary voltage tap settings (usually +/- 5% in 2.5% increments) to bump the no-load voltage up if you anticipate heavy, continuous loading.
3. True Power Factor Limits
Transformers are rated in kVA (apparent power), not kW (real power). The chart assumes you are managing your power factor. If your facility has a heavy inductive load (large uncorrected motors) resulting in a 0.70 power factor, a 75 kVA transformer can only safely deliver 52.5 kW of real work before the windings overheat from the reactive current. Always measure or estimate your site power factor before finalizing the kVA size.
Final Selection Protocol
Do not leave your transformer selection to guesswork or let the supply house ship whatever is sitting on their pallet. Follow this exact sequence for a standard commercial installation:
- Calculate Base kVA: Sum your 3-phase kW loads, divide by 0.85 PF, and round up to the nearest standard size (e.g., 62 kW / 0.85 = 72.9 kVA → Select 75 kVA).
- Check the Environment: If ambient temp is under 40°C and altitude is under 3,300 ft, no derating is required.
- Audit the Loads: If non-linear loads exceed 30%, upgrade the spec to a K-13 unit. Otherwise, stick to standard.
- Order the Default Pick: For 90% of standard commercial applications, order a 75 kVA, 3-phase, 60Hz, 480V Delta to 208Y/120V, 150°C rise, 5.75% impedance, NEMA ST-20 compliant dry-type transformer (e.g., Eaton V12T75C or equivalent Schneider Square D model).
- Set the Taps: Upon installation, verify primary voltage. If it reads 490V or higher, leave taps at nominal. If it reads 465V or lower, adjust the primary taps down by 2.5% to maintain secondary voltage under load.
